The present invention is directed to methods of drawing a steel wire. Specifically, the wire is subjected to a non-linear method of drawing a steel wire, resulting in an increased strength of the wire.
It is frequently desirable to reinforce rubber articles (such as, tires, conveyor belts, power transmission belts, timing belts and hoses) by incorporating therein steel reinforcing elements. Pneumatic vehicle tires are often reinforced with cords prepared from brass-coated steel filaments. Such tire cords are frequently composed of high carbon steel or high carbon steel coated with a thin layer of brass. Such a tire cord can be a monofilament, but normally is prepared from several filaments that are stranded together. In most instances, depending upon the type of tire being reinforced, the strands of filaments are further cabled to form the tire cord. It is important for the steel alloy utilized in filaments for reinforcing elements to exhibit high strength and ductility as well as high fatigue resistance.
Transformation of the steel alloy into a filament suitable for reinforcing rubber articles involves multiple processing stages, including rough drawing, patenting, brass plating and fine drawing. The selected process to achieve a steel wire with defined characteristics can include many variations on those processing stages, including repeating the different stages.
Typically, rough drawing, i.e. dry drawing of a rod to an intermediate wire diameter, is accomplished by using a taper draft. In a taper draft, larger diameter reductions are made at the beginning die positions while the wire is ductile, i.e. a relatively high drawing strain is used, and at the final die position, smaller reductions are made, i.e. a relatively lower drawing strain is employed, when the wire has a higher strength due to strain hardening. Conventional linear taper drafts are designed to achieve equal work done at the first and the last die position, and the dependence of the strain on the die position represents a straight line. In this approach, only original wire strength and the final strength are taken into account, while the wire strength at the intermediate die positions is not considered. Thus the amount of drawing strain employed through the die positions is reduced by a constant amount as the wire diameter is reduced. FIG. 3 illustrates the drawing strain and die position relationship for a linear taper draft. Such linear tapered drawing is only used during rough drawing.
Another known method of drafting is an even area reduction draft. During even area reduction, the drawing strain applied at each successive die in the die path is the same as the diameter of the wire is slowly reduced. Even area reduction is employed during both rough and fine drawing.
The invention provides solutions for designing optimized die drawing drafts to achieve increased efficiency of the drawing process and high strength wires with improved torsion characteristics. The invention takes into account the actual wire strength at the intermediate die positions while drawing a wire to a final desired diameter.
Disclosed is a process for forming a drawing of a wire to smaller diameter, either an intermediate bright wire size or a final desired diameter.
Disclosed is a process for drawing a wire to a desired diameter comprising the steps of selecting a wire having an initial diameter and drawing the wire through a series of wire dies to reduce the wire diameter to a desired diameter wherein the drawing strain at each successive die is gradually reduced. This method of drawing is referred to as a non-linear tapered draft.
In one aspect of the invention, the drawn wire has a final desired diameter of about 0.1 to about 0.4 mm. Such a diameter range is exemplary for a final diameter after both a rough draw, patenting, and a final drawing. Alternatively, if the drawing of the wire by means of the non-linear tapered draft is the rough drawing, than the final desired diameter is about 2.5 to about 1.0 mm. Both of these diameters are preferably for an initial wire diameter is about 4.0 to about 5.5 mm and such wires are most useful in tire manufacturing, automotive part manufacturing, and conveyors belts.
Also disclosed is that the wire may be drawn again after the non-linear tapered draft; thus the non-linear tapered draft occurs during the rough draw. The additional drawing following the rough draw may be a skin pass wherein the diameter is reduced by less than 4% or it may be a desired fine draw.
If the additional draw is the fine drawing of the wire done to the final wire diameter, the draft technique used may be selected from among the following drawing methods: gradually reducing the drawing strain at each successive die (i.e., non-linear tapered draft); reducing the drawing strain at each successive die by a constant amount (linear tapered draft); applying a constant drawing strain at each successive die (even area reduction); or a combination of any of the above.